Electric power demand for aircraft continue to increase. Indeed, some aircraft demand relatively high power requirements—on the order of 1 megawatt—throughout the flight envelope. Even at relatively lower electric power demands, a traditional approach is to avoid encumbering the gas turbine engines responsible for providing thrust to the aircraft by using a separate, dedicated gas turbine engine, also known as an Independent Power Unit (IPU) or Auxiliary Power Unit (APU), to address the need for electric power generation. The use of an IPU/APU resolves the challenges of simultaneously managing the variation in electric power demand and the variation in propulsion power demand.
In a distributed propulsion architecture, with auxiliary fans relying on electrical power, the demand for electrical power generation (Pe) increases. As the ratio of power for electrical power generation (Pe) relative to the power for aircraft propulsive power generation for thrust (Pt) increases, the challenge of meeting both requirements (i.e., Pe and Pt) with a propulsion engine becomes increasingly difficult. This is because varying the power extraction from either the high-pressure spool and/or the low-pressure spool to drive a generator can detrimentally impact the stable operating range of the compressor. While the IPU/APU addresses certain challenges in delivering electric power, it adds significant cost, weight, and complexity to the aircraft system. Moreover, with the increase in electrical power demand at relatively high altitudes, the size, weight, and cost of the IPU/APU becomes increasingly prohibitive.
In addition, the increased number of electrical components in newer aircraft emit relative large amounts of heat that should be transported away from the components. A concept that has recently been developed is a turbofan engine configured with two separate, concentric bypass streams, with the outermost stream being designated as the “3rd stream.” In some cases, the 3rd stream air passes through all the fan stages (if more than one), and in other cases it may only pass through a subset of the stages. This air, like the traditional turbofan bypass air, bypasses the core of the engine. The 3rd stream air is sufficiently pressurized, but is also low enough in temperature, to provide effective cooling for the electrical components. Although this 3rd stream configuration provides additional cooling, it also exhibits certain drawbacks. For example, it increases the diameter of the engine relative to the baseline turbofan engine that provides only thrust without the additional cooling flow. This increase in diameter is detrimental to aircraft size and weight, especially in aircraft with embedded engines.
Hence, there is a need for an improved system that not only provides for high electric power extraction from the gas turbine propulsion engine, but also provides the component cooling air and the total propulsive thrust needed by the aircraft in a more compact, lighter weight configuration. The present invention addresses at least this need.